In the petroleum and petrochemical industries, acid gases containing CO2, H2S, and VOCs are commonly scrubbed with aqueous caustic solutions that can contain, for example, NaOH. Such scrubbing processes yield a hazardous spent caustic solution having a high pH, typically greater than 10.5, and sulfide reactivity. Spent caustic solutions are also characterized by high chemical oxygen demand (COD) and biochemical oxygen demand (BOD). BOD and COD are measures of oxygen uptake by water arising from chemical and biodegradable agents in the water. The COD and BOD exerting compounds in spent caustic solutions can include, but are not limited to sodium sulfide (Na2S) sodium hydrosulfide, and various organic compounds, including naphthenic acids and the like.
Typically, sulfides in the spent caustic solutions are oxidized using a gaseous oxidant such as air or oxygen to convert the sulfides into thiosulfates and sulfates which can be subsequently neutralized using acid. Unfortunately, conventional oxidation can result in severe foaming problems, rendering the air oxidation process inoperable without expensive defoaming chemicals. Conventional efforts to control pH before oxidation by, e.g., adding sulfuric acid, undesirably evolves hydrogen sulfide gas.
The art is in need of alternative methods for treating spent caustic solutions to avoid foaming and hydrogen sulfide evolution while yielding a waste discharge stream having reduced BOD and COD. It is also desirable to reduce the amount of organic matter present in treated discharge streams.